This invention relates to a process for the production of maleic anhydride and particularly to the separation of resulting mixtures of maleic anhydride and acrylic acid by distillation so as to inhibit polymer formation.
In the process for producing maleic anhydride from butane or benzene by oxidation with air in the vapor phase, the resulting crude product contains not only the maleic anhydride but also carbon monoxide, carbon dioxide, steam, residual air, acetic acid, acrylic acid and various other heavy end products. The constituents which are noncondensable at the temperatures involved including the carbon monoxide, carbon dioxide, steam and residual air are vented. The condensables including the acetic acid, acrylic acid, the heavy ends and the maleic anhydride product are then condensed or extracted into a solvent such as water or organic solvents. The light ends, i.e. the acetic acid and the acrylic acid are distilled off from the maleic anhydride product and the heavy ends. In this distillation, the acrylic acid, which is now concentrated along with the acetic acid, can polymerize to form gums, resins and even solids which can plug the tower internals and transfer lines.
Hydroquinone monomethyl ether activated by dry air (oxygen) would normally be the inhibitor of choice to prevent this polymerization since it is used commercially when purifying crude acrylic acid to prevent polymerization. However, it has been found that its effectiveness as a polymer inhibitor is diminished when maleic anhydride is present, apparently due to the formation of a stable equimolar complex with maleic anhydride. Therefore, it was necessary to maintain the acrylic acid concentration of the distillate below 30 wt. % for the inhibitor to be effective. Dilution of the distillate is achieved by allowing the maleic anhydride concentration to increase, but this constitutes a loss of valuable product.